Mens' shirt patterns vary in how the collar is assembled. Some collars are made in one piece with no collar stand. These are sometimes referred to as a "camp" style collar. They do not fold over as nicely, in my opinion, as a two-piece collar; because of this, I tend to look for two-piece collars when I am selecting men's shirt patterns.

The first step is to assemble the collar portion. You will use two collar pieces along with an interfacing piece. Be sure to mark the center of the collar pieces and any other notches before proceeding with the next step.

Look at your collar pieces and decide which piece you want to have showing on top and mark the other piece with a pin. This piece is now your undercollar. Apply interfacing as desired to the undercollar (trim the collar points off the interfacing piece to reduce bulk in the seam).

When making a shirt collar, there is a little trick that makes turning and pressing the collar easier. Trim the undercollar piece slightly (about 1/16 of an inch) along the edge opposite the notches. On the shorter sides, trim 1/16 inch away at the tips as well, but taper the trimming out as you go toward the notched edge (refer to photo).

Pin the collar pieces together, stretching the undercollar slightly to match up the points and side seams. This is one time where you really need to pin the pieces so that they alight correctly all the way around. The pieces will be tight - this tension will help the collar roll over so the seam doesn't show from the right side. Sew around the sides and along the un-notched edge using the pattern recommended seam allowance. As you approach the last inch before the point, reduce your stitch length. When you reach the corner, rather than turning on the point and immediately sewing in the other direction, turn your collar halfway and sew one or two small stitches across the point end and then turn the collar the rest of the way to sew the other side. This gives the area a bit more room for the turned seam allowance later.

When you are done sewing, the collar points may curl up slightly; this is okay.
Press seam flat.

Grade the seam, trimming the undercollar and interfacing down to 1/4 inch. Trim the seam at both sides of the collar point down to 1/8 of an inch (if your fabric is lightweight or frays easily, only trim to 1/4 inch to reduce the chances of the fabric at the point tearing out of the seam).

If you have a point presser, press the collar seam open before turning the collar points. A point presser is a great tool that can help you press seams open in tight areas.

If you don't have a point presser (yet), there is a way to press the majority of the seam open from the inside. First, gently turn the collar points right side out. The best tool to use for this is a point turner, but a chopstick works pretty well, if you don't happen to have a point turner handy.

If your points are being stubborn and not turning well, you can use a straight pin to gently pull the point out. If you do this, be very careful to not pull the fabric out of the seam allowance.

Once you have both points turned to your satisfaction, lay the collar, right side down, on your pressing board and open up the center so it looks like a little boat (see photo). You can now take the tip of your iron and press open the seam along the outer edge of the collar. You won't be able to get all the way to the point, but even pressing this much is very helpful when you finish turning and pressing the collar.

After you have pressed the seam open from the inside, lay the collar flat, with the undercollar facing up. Press the collar flat, rolling the seam over to the undercollar side, and match up the notched edge.

Baste the notched edge together.

Top stitch around the sewn sides of the collar 1/4 inch from the edge.